Practicing position discrimination improves visual performance in children with amblyopia. In a previous study, we found a 30% improvement in position acuity after 7–10 sessions of practice, in children who previously had completed occlusion therapy before starting our experiment. The present study was aimed at quantifying the limits and time course of perceptual learning in previously untreated juvenile amblyopia.

Two children with untreated amblyopia (line-letter acuity: AL 20/100−2, SG 20/125+2) practiced a positional acuity task repetitiously. The task was to judge which of three pairs of segments was misaligned. Each segment consisted of eight discrete Gabor patches (carrier SF, 5 cpd). The observer was trained at three or four positional noise levels (including zero). Viewing was monocular. Trial-by-trial feedback was provided. More than 10000 trials were performed over a 6-week period.

After practice, the observers showed substantial improvement in positional discrimination. Positional acuity improved on average by about 67%, both with and without noise. The improvement began to plateau after 20 sessions. In observer SG, the performance in the amblyopic eye following practice was almost comparable with that in the non-amblyopic eye. The improvement could be attributed primarily to increased efficiency (AL 400%, SG 1600%) with a small decrease in equivalent internal noise.

Our findings suggest that the “fresh” (previously untreated) amblyopic brain is remarkably plastic; positional acuity can be substantially improved after 40 hours of training. We speculate that perceptual learning techniques may add an effective new clinical method for the treatment of juvenile amblyopia.